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Message-ID: <325768cd-19bd-71ae-83d6-1ca5e84f7462@redhat.com>
Date: Tue, 14 Jun 2022 14:22:40 -0400
From: Waiman Long <longman@...hat.com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] mm/kmemleak: Prevent soft lockup in first object
iteration loop of kmemleak_scan()
On 6/14/22 13:27, Catalin Marinas wrote:
> On Tue, Jun 14, 2022 at 06:15:14PM +0100, Catalin Marinas wrote:
>> On Sun, Jun 12, 2022 at 02:33:01PM -0400, Waiman Long wrote:
>>> @@ -1437,10 +1440,25 @@ static void kmemleak_scan(void)
>>> #endif
>>> /* reset the reference count (whiten the object) */
>>> object->count = 0;
>>> - if (color_gray(object) && get_object(object))
>>> + if (color_gray(object) && get_object(object)) {
>>> list_add_tail(&object->gray_list, &gray_list);
>>> + gray_list_cnt++;
>>> + object_pinned = true;
>>> + }
>>>
I may have the mistaken belief that setting count to 0 will make most
object gray. Apparently, that may not be the case here.
>>> raw_spin_unlock_irq(&object->lock);
>>> +
>>> + /*
>>> + * With object pinned by a positive reference count, it
>>> + * won't go away and we can safely release the RCU read
>>> + * lock and do a cond_resched() to avoid soft lockup every
>>> + * 64k objects.
>>> + */
>>> + if (object_pinned && !(gray_list_cnt & 0xffff)) {
>>> + rcu_read_unlock();
>>> + cond_resched();
>>> + rcu_read_lock();
>>> + }
>> I'm not sure this gains much. There should be very few gray objects
>> initially (those passed to kmemleak_not_leak() for example). The
>> majority should be white objects.
>>
>> If we drop the fine-grained object->lock, we could instead take
>> kmemleak_lock outside the loop with a cond_resched_lock(&kmemleak_lock)
>> within the loop. I think we can get away with not having an
>> rcu_read_lock() at all for list traversal with the big lock outside the
>> loop.
> Actually this doesn't work is the current object in the iteration is
> freed. Does list_for_each_rcu_safe() help?
list_for_each_rcu_safe() helps if we are worrying about object being
freed. However, it won't help if object->next is freed instead.
How about something like:
diff --git a/mm/kmemleak.c b/mm/kmemleak.c
index 7dd64139a7c7..fd836e43cb16 100644
--- a/mm/kmemleak.c
+++ b/mm/kmemleak.c
@@ -1417,12 +1417,16 @@ static void kmemleak_scan(void)
struct zone *zone;
int __maybe_unused i;
int new_leaks = 0;
+ int loop1_cnt = 0;
jiffies_last_scan = jiffies;
/* prepare the kmemleak_object's */
rcu_read_lock();
list_for_each_entry_rcu(object, &object_list, object_list) {
+ bool obj_pinned = false;
+
+ loop1_cnt++;
raw_spin_lock_irq(&object->lock);
#ifdef DEBUG
/*
@@ -1437,10 +1441,32 @@ static void kmemleak_scan(void)
#endif
/* reset the reference count (whiten the object) */
object->count = 0;
- if (color_gray(object) && get_object(object))
+ if (color_gray(object) && get_object(object)) {
list_add_tail(&object->gray_list, &gray_list);
+ obj_pinned = true;
+ }
raw_spin_unlock_irq(&object->lock);
+
+ /*
+ * Do a cond_resched() to avoid soft lockup every 64k
objects.
+ * Make sure a reference has been taken so that the object
+ * won't go away without RCU read lock.
+ */
+ if (loop1_cnt & 0xffff) {
+ if (!obj_pinned && !get_object(object)) {
+ /* Try the next object instead */
+ loop1_cnt--;
+ continue;
+ }
+
+ rcu_read_unlock();
+ cond_resched();
+ rcu_read_lock();
+
+ if (!obj_pinned)
+ put_object(object);
+ }
}
rcu_read_unlock();
Cheers,
Longman
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